This invention relates to a radar antenna system. In particular, the invention relates to an active antenna array of the kind which can be scanned electronically in order to effect tracking of a target position in space. The invention is applicable to tracking and multifunction radar apparatus which might be used in weapon systems, microwave landing systems, airborne and space based radar apparatus, for example.
The conventional radar apparatus in performing a tracking function frequently makes use of a circularly symmetric reflector system which creates a pencil beam which is steered mechanically in azimuth and elevation. In an alternative construction, a passive phased array has a single lumped transmitter whose power output is fed to the elements of an electronically steerable antenna array. The feeder arrangement includes high power RF signal networks including phase shifters for transmission and a similar network for the receive function. The antenna array is then capable of producing a single RF signal beam or multiple beams in space.
Both the mechanically steerable and the electronically steerable antennas can suffer from the effects of reflections or multipath signals from the ground or sea surface between a tracked target and the radar apparatus. The effect of these multipath signals is to degrade the capability of the apparatus to track the image of the target arising from the reflection.
Various techniques have been used in attempts to overcome the problem. One proposed technique is to shape the tracking beam in order to minimise the proportion of the radar energy transmitted in the direction of the land or sea surface. Any improvement in performance is limited by the antenna aperture available and whatever freedom there might be to shape the pattern sufficiently without losing the tracking performance of the radar system.
An alternative technique is to use a very wide band operation. This possibility may not be favoured because of the very wide bandwidth which may be necessary to decorrelate the wanted and unwanted signals, and the cost of the hardware needed to achieve the wide bandwidth.
The present invention was devised to provide a further alternative in which phase differences between the wanted and unwanted signals are able to be used.
According to the invention, there is provided in a radar antenna system comprising an active antenna array formed of antenna elements each of which is connected directly to a respective RF transmit/receive module, the method of operation which comprises considering a particular block of one or more elements within the array as forming a first element group, considering a second block of elements as forming a second element group, transmitting a radar signal from one of said element groups, receiving a return signal at the same or a different one of said element groups, and switching a radar receiver electronically between said first and said second element groups so as to produce a phase difference between a direct and a multipath reflection of the transmitted signal, said phase difference enabling radar information from a direct reflection to be distinguished from that due to a multipath reflection.
Preferably, two or more element groups of the array are arranged such that the antenna phase centre is movable vertically as well as horizontally within the array area. The element groups may be arranged such that the antenna phase centre is able to be rotated circularly or elliptically within the array area.
In a different embodiment, there is provided a radar antenna system comprising antenna elements arranged in an active array, the elements being located in predetermined groups such that an individual group can be selected electronically from within the whole array aperture, the said selection being effected by variable attenuator means. The variable attenuator means may comprise a RF signal feed network or a switch.